ABSTRACT Glioblastoma multiforme (GBM) is the most common malignant central nervous system cancer in adults. Conventional treatment consisting of tumor surgical resection followed by photon radiation in combination with adjuvant chemotherapy results in a median survival of 14.6 months. Therapeutic advancements have been limited by the extreme chemo- and radio-resistance of GBM. Combination photon and proton therapy has been found to increase survival time, but complications due to radionecrosis have prevented the implementation of this therapy regimen. Addition of a radiosensitizing agent could allow the use of lower radiation doses to achieve tumor cytotoxicity while sparing normal tissue. This application aims to investigate the use of hydrogen sulfide (H2S) as a novel radiosensitizing agent for the treatment of GBM. H2S has been identified as the third endogenous gasotransmitter and sodium sulfide (Na2S), DATS and AP39 are used experimentally to increase cellular H2S. Our preliminary studies suggest acute, high doses of Na2S significantly induce oxidative DNA damage and can kill GBM cell lines in culture, while sparing normal cerebral endothelial cells. Na2S also altered GBM mitochondrial function, increased protein acetylation, increased oxidized glutathione in GBM cell lines and radiosensitized GBM cells to proton or photon ionizing radiation. Our hypothesis is that H2S enhances ionizing radiation-selective killing of GBM cells by increasing reactive oxygen species (ROS) production from the mitochondria, enhancing DNA damage and leading to hyper-activation of poly(ADP-ribose) polymerase (PARP). We further hypothesize that hyper-activation of PARP decreases cellular NAD+ levels, inhibiting HDAC/ sirtuins, resulting in cell death due to inhibition of DNA repair and higher DNA damage levels. This hypothesis will be tested in two aims using two GBM cell lines, an astrocyte cell line, a neuronal cell line and a cerebral microvascular cell line. Aim 1 will test whether Na2S, DATS and AP39 enhance only GBM proton or photon ionizing radiation cell killing. We will probe the mechanism by examining ROS, DNA damage production, and mitochondrial dysfunction. MitoTEMPOL, a mitochondrial antioxidant, and two GBM rho zero cell lines deficient in oxidative phosphorylation will also be used to determine if the ROS is generated in the mitochondria. Aim 2 will test whether Na2S-induced radiosensitization is through hyper-activation of PARP and reduced DNA repair. Upon completion of this work, we will understand why H2S radiosensitizes GBM cells and know which H2S-releasing compound is the best radiosensitizer of GBM cells. This will potentially open avenues for development of H2S-releasing compounds as radiosensitizers. Work in this proposal will be performed by graduate, undergraduate and high school students. These students will learn about radiation biology, oxidative stress and DNA repair and will gain novel insight into radiotherapy and the use of the ProteusONE pencil beam. This proposal meets the R15 goals of enhancing education of students in Shreveport. Funding will significantly improve the ability of the PI to perform these studies, and will enhance the institutional graduate program.